1. Field of the Invention
The present invention relates generally to apparatuses and methods for recording information in an optical storage medium, and to multi-layer optical storage media. More specifically, this invention relates to an apparatus and a method for recording information in an optical storage medium using a pulsed laser of which a light intensity is adjustable, and a multi-layer optical storage medium in which information is recorded using such a pulsed laser.
2. Description of Related Art
Optical storage media, such as DVD-R and Blu-ray (registered trademark) discs, in which information is retrievably recordable, typically adopts the dual-layer recording scheme in order to increase the storage capacity. In recent years, to enable recording in more than two layers, a recording method using a two-photon absorption compound has become a focus of study (see JP 2006-48832 A).
The two-photon absorption compound is a compound which absorbs light with electrons excited therein only when two photons simultaneously strike the compound. Therefore, the two-photon absorption compound induces reaction (absorption) with a probability proportional to the square of the intensity of the input light and with high resolution and selectivity in depth of focus. To be more specific, when a laser beam is focused into the optical storage medium having a recording layer containing a two-photon absorption compound, the two-photon absorption compound absorbs (reacts with) the input beam only in a small region around a focused point and never reacts in regions even slightly out of the small region in depth (in the direction of travel of the laser beam). In this way, the two-photon absorption compound is capable of inducing reaction in a small region in depth, and thus a recording layer made of this compound is suitable for recording information in multiple layers.
It is desirable to use a pulsed laser such as a femtosecond laser instead of a hitherto prevalently used continuous-wave laser particularly when an optical storage medium containing a two-photon absorption compound is used to record information therein. This is because the pulsed laser can produce a laser beam having a peak power sufficient to induce two-photon absorption reaction even when it operates at a relatively low average power.
As a pulsed laser for illuminating an optical storage medium, however, a solid-state laser would intrinsically involve a difficulty in having its own output power regulated rapidly. On the other hand, recording of information in a multi-layer optical storage medium would inevitably entail a loss due to absorption (in each layer) and reflection (between adjacent layers) of a laser beam while traveling through the layers deep into an intended layer in which information is to be recorded. Accordingly, it would be desirable to render the intensity of a laser beam for recording changeable according to the depth (of hierarchy) of the intended recording layer. Moreover, if a compound which may be caused by application of light to change into a state with a fluorescing property is adopted as an optical recording material, an excessively high concentration of the fluorescent material produced by application of light during the recording process would possibly result in a lower intensity of the induced fluorescence. With this in view, it would be desirable that the intensity of light to be emitted for recording be adjustable.
The present invention has been made in an attempt to address the above-described problems.